This invention relates to a method of treating fine coal refuse usually in the form of an aqueous slurry in order to be able to dispose of the refuse and water in a safe and economical manner.
One of the major problems in modern coal mining and cleaning is the safe and economical disposal of fine refuse, which usually has a size of less than about 28 mesh (and more commonly less than about 48 mesh). A large portion of this refuse is mineral matter and thus the refuse has poor fuel value.
Up until recently, this material has been disposed of by one of several methods. Pumping of an aqueous slurry of the refuse behind a permanent impoundment was commonly used. Construction costs were minimized by using coarse coal refuse to build the impoundment or dam. Operating costs were very low. However, this method as commonly practiced was found to be very unsafe. For this reason, state and federal regulations now provide stringent rules governing construction of impoundments. Thus, today this method is a very expensive disposal method.
Another proposed disposal method has been to dig small ponds into which the refuse slurry is dumped. The solids settle to the bottom and the water is pumped off. When the pond is full, it must then be excavated and hauled off for permanent disposal. The common method is to haul the excavated material to the coarse refuse dump and mix the two materials together. However, digging and handling characteristics of the settled fine coal refuse are generally very bad. This results in a very inefficient, messy and expensive disposal method.
Another proposed method is mechanical dewatering, either by vacuum filters, pressure filters or centrifuges, in place of gravity dewatering of the holding ponds. While water content of the filter or centrifuge cakes is usually lower than the pond-settled solids, few installations can produce cakes with satisfactory handling and disposal characteristics. The disposal problems and economics are thus very similar to those in the temporary ponding procedure.
A method that has been developed recently is the addition of a stabilizing additive to the fine coal refuse slurry to cause solidification. Lime alone or lime and blast furnace slag are stabilizing additives that have been used for this purpose. See U.S. Pat. No. 4,015,997, which is incorporated herein by reference. These stabilization methods have improved the stability of the fine coal refuse, thereby enabling improved and more efficient handling of the stabilized fine coal refuse. However, when the slurry to which the additive has been added is placed in a settling pond, very long time periods such as several days to two or three weeks are often required to reach a suitable stability. This requires very large settling ponds which increase expense.
Also the possibility of combining dewatered fine coal refuse with the additive and transporting directly to the land fill has been suggested. This would eliminate the need for a settling pond. However, to date this has not proven to be commercially successful.
U.S. Pat. No. 3,870,535 suggests the use of lime alone or lime and fly ash as an additive to a combined coarse and fine refuse. While reduced setting times have been achieved with the coarser material, there is no suggestion that use of these additives for fine coal refuse alone would improve the stabilization rate.